Diodes
The simplest way of describing a diode is a single P-N junction with a lead attached to each end. The end with the N-type material is named the Cathode, and the end with the P-type material is known as the Anode. Diodes (and other semiconductor devices) behave differently than simple resistors due to the fact they are non-linear, which means their current is not directly proportional to their voltage. When you have a simple resistive circuit, current proportional to voltage is plotted on a straight line, and is therefore linear. The graph of a diode has a certain point where it begins to conduct, and also a reverse point where it starts to breakdown.
Starting with the forward region, once the biasing voltage source overcomes the barrier potential, the diode begins to allow electron flow. For a normal doped silicon diode, this is .7 volts. This is also known as the knee voltage, because once .7 volts is achieved, the voltage on the graph turns very sharply up, creating what looks like a knee in the line. Above the knee voltage, diode current increases very rapidly. Once the barrier potential is overcome, all that impedes the flow of current is the resistance of the P and N junctions. This is called the Bulk resistance of the diode and can be calculated from the sum of the resistance of the P and N junctions.
Another thing to consider in the forward region is the maximum DC forward current. This can be found on datasheets. Once this is achieved, the diode will probably be destroyed due to excessive heat. This is usually termed If(max) or Io. Diode datasheets also have a maximum power disspation rating.
When diodes are operated in the reverse region, you get a very small amount of leakage current, and there is a point when the diode will breakdown, due to an effect called avalanche. When so many electrons are being forced onto the diode, the energy that propels them is enough to force other electrons out of their valence band and across the P-N junction. This breakdown voltage is also put on the datasheet. Although some specialty diodes, like zener diodes are meant to be operated in this way, on a normal diode, avalanche is to be avoided.
Special purpose diodes
Rectifier diodes are constructed to allow current in only one direction. when used with an AC voltage source, this cuts off one side of the sine wave, and creates a pulsating DC wave. Say the circuit is connected so only the positive alternations are passed, once the sine wave reaches 0 volts, it remains there until the wave reaches 0 volts again, and then continues on passing a positive sine.
This arrangement of a diode in a circuit is known as a half-wave rectifier.
where:
Vp is your peak voltage
Vs is your voltage source
and .7v is the voltage drop across the diode (silicon).
If you arrange a diode this way on both ends of a AC voltage circuit and then combine them, you get what is known as a full-wave rectifier. Only the positive waves are passed, and they are 180 degrees out of phase with each other. The end effect is as the positive sine wave of the first signal drops to zero, the other side pulses and completes it’s sine, and so on. the result ends up looking like a regular sine wave, with the negative alternations flipped positive. Full-Wave rectifiers can be used in power supplies, where an AC signal is provided and a DC voltage is desired. Full wave rectifiers must use a center tapped transformer.
Since the negative alternations are simply dropped, normal full-wave rectifiers are wasteful. When designing a rectifier circuit, it is better to use a bridge rectifier. Bridge Rectifiers have two ways for the current to flow, so there is a path on each alternation. Most power supplies use this configuration. Since a center tap is not needed, the rectified voltage is twice what a full wave recifier would create.
In bridge rectifiers, another thing to consider is since you have two diodes dropping voltage on each path, the voltage is calculated by:
where:
Vp is your peak voltage
Vs is your voltage source
and 2(.7v) is two .7 voltage drops across the diodes (silicon).
If you connect a DC Voltmeter across the load, it will indicate the average value of the full wave signal, which is:
which is equivalent to
.636 * peak voltage.
The frequency of a full wave signal is double the input frequency, since a waveform completes it’s cycle as soon as it repeats. For a 60 hertz input:
Time = 1/Frequency
Time = 1/60
Time = 16.7ms
The rectified voltage has a period of
Time2 = 16.7ms/2
Time2 = 8.33ms
Frequency2 = 1/8.33ms
Frequency2 = 120 hertz.
Another way to put this simply is to say:
Fout = 2Fin
where:
Fin is Frequency In
Fout is Frequency Out
Another type of diode is the Zener diode. Most diodes are never operated in the breakdown region beacuse it would damage them. A zener is manufactured to be operated in the reverse region, and to have a specific voltage where it will begin to conduct. Zener’s are available in many different voltages. A zener diode is sometimes referred to as a zener voltage regulator becuase they can be used in parallel to allow a certain voltage to pass to the load, and then begin to conduct once the zener voltage is reached, therefore passing the remaining voltage through the zener and bypassing the load. A series resistor is always used in this configuration to limit current flow.
Maximum power through a zener diode is found by:
Zener Impedance can be found through:
The change in Zener voltage (^Vz) can be found by:
LED‘s or light emitting diodes are another specialty diode. As the electrons cross the P-N junction and fall into holes, they radiate energy. LED’s are constructed to show this as visible light. By using elements like arsenic and phosphorus, LED’s can be manufactured in red, green, yellow, blue, orange and even infrared. The exact voltage drop across LED’s depends on the color. The typical voltage drop is 1.5 to 2.5 volts for currents between 10 and 50 milliamps.
All diodes have an associated capacitance, due to the way they are constructed. The P and N regions can be thought of as the plates, and the depletion region is the dielectric. Varactor diodes are built to take advantage of this, and are used in tuning circuits where a variable resonant frequency is desired. As the voltage is varied, the depletion region expands and contracts, causing the capacitance to change. You can connect a varactor in parallel with an inductor to get a resonant circuit, and then vary the biasing voltage to achieve specific resonant frequencies.
